Method for repairing a blade

ABSTRACT

A method for repairing a blade in a gas turbine engine comprises the steps of: isolating the damage on the airfoil of the blade; forming a cut back in the shape of elongated “D” shaped recess with a pair of fillets, a depth and a longitudinal axis of the “D” shaped recess having a length along the leading or trailing edge of the airfoil; and the fillets having a respective radius.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority on U.S. Provisional ApplicationSer. No. 61/838,022, filed on Jun. 21, 2013.

TECHNICAL FIELD

The described subject matter relates generally to gas turbine engines,and more particularly to a method for repairing a damaged blade.

BACKGROUND ART

Compressor blades of gas turbine engines are subject to foreign objectdamage (FOD). The nature of the damage could vary depending on the typeof the foreign object: nicks, tears, dings and blade bending are commontypes of damages seen in the field. In order to make the damaged bladesflight worthy again, the damaged areas of the airfoil are repaired in awell-defined fashion as outlined in repair and overhaul manuals. Atypical blade repair scheme involves a cut out in the area of interestthat is in the shape of an arc or “C” shape.

The typical blade repair scheme is not always successful because peaksteady stress and peak vibratory stress locations may both coincide atthe cutback radius. The peak vibratory stress may correspond to aresonance condition. This coincidence of vibratory and steady stresspeaks is a concern from a durability stand point.

There is a need to improve such repair methods.

SUMMARY

In accordance with the present disclosure, there is provided a methodfor repairing a blade in a gas turbine engine comprising: identifying adamage on an edge of an airfoil of the blade; forming a cutback aroundthe damage in the edge, the cutback shaped to comprise at least a pairof fillets r₁, r₂ in the edge on opposite ends of the cutback, a depth dfrom the edge, and a length l along the edge.

Further in accordance with the present disclosure, there is provided ablade in a gas turbine engine comprising: an airfoil having a leadingedge and a trailing edge; and a cutback machined in at least one edgeamong the leading and trailing edges at a location of damage, thecutback comprising a shape defined by at least a pair of fillets r₁, r₂on opposite ends of the cutback, a depth d from the edge, and a length lalong the edge.

Still further in accordance with the present disclosure, there isprovided a gas turbine engine comprising: at least one blade having aleading edge and a trailing edge; and a cutback machined in at least oneedge among the leading and trailing edges at a location of damage, thecutback comprising a shape defined by at least a pair of fillets r₁, r₂on opposite ends of the cutback, a depth d from the edge, and a length lalong the edge.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures in which:

FIG. 1 is a schematic view of a longitudinal section of an embodiment ofa turbofan gas turbine engine;

FIG. 2 is a fragmentary perspective view of a blade repaired with aconventional “C” shaped cutback;

FIG. 3 a is a graphical representation of FIG. 2 showing the peakvibratory stress;

FIG. 3 b is a graphical representation showing the peak steady stress;

FIG. 4 is a fragmentary perspective view of a blade repaired inaccordance with an embodiment of the present disclosure;

FIG. 5 a is a graphical exemplary representation of FIG. 4 showing thepeak vibratory stress on the blade of FIG. 4;

FIG. 5 b is a graphical exemplary representation showing the peak steadystress on the blade of FIG. 4;

FIG. 6 a is a schematic view of another shape of the cutback of FIG. 4;and

FIG. 6 b is a schematic view of another shape of the cutback of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically depicts a turbofan engine 10 which, as an example,illustrates the application of the described subject matter. Theturbofan engine 10 includes a nacelle 11, a fan 12, a compressor module14, a combustor module 16 and a high pressure turbine module 18.

FIG. 2 of the prior art shows a typical compressor disc 20 with anairfoil 22, a leading edge 24 a trailing edge 26, and hub 27. As shownin FIG. 2, a repair in the form of a conventional “C” shaped cutback 28is applied to a mid-span area of the leading edge 24. As shown in thegraphs represented in FIGS. 3 a and 3 b, the peak vibratory stress andthe steady stress peaks may coincide at the mid-span area where therepair 28 is made. This may be a cause for concern of reduceddurability.

FIG. 4 shows a similar compressor disc 30 having an airfoil 32, aleading edge 34 and a trailing edge 36. A repair has the form of a “D”shaped cutback 38 (hereinafter referred to as “D” shaped for simplicity.The “D” shaped cutback 38 may be compared to an elongated recessresembling a geometric form between a rectangle and an ellipse. It ischaracterized by fillets r₁ and r₂. The radii of the fillets r₁ and r₂may or may not be equal in value. It may be possible to use the sametooling if the radii of the fillets r₁ and r₂ is equal. In anembodiment, the fillets r₁ and r₂ may be spaced apart by a generallystraight cutback edge f. By generally straight, it is understood thatthe cutback edge f may be substantially straight, or may have a radiusthat is substantially greater than the fillet r₁ and r₂, i.e., bequasi-straight. It is also considered not to have any edge spacing apartthe fillet r₁ and r₂, whereby 1=r₁+r₂, in a limit case for the cutback38 which would have more of a “C” shape in this limit case.

Still referring to FIG. 4, the length l and depth d will vary dependingon the damage to be repaired. Fillets r₁ and r₂ may vary as a functionof the depth d. For instance, an appropriate ratio range for l/d is 1 to20, while r₁/d=0.2 to 20 and r₂/d=0.2 to 20. The depth d is within themaximum blend limit.

For example, in proposed applications the length l may be between 0.060″and 3.0″, for d between 0.030″ and 1.5″, and for r₁, r₂ between 0.030″and 1.5″.

Referring now to FIGS. 5 a and 5 b, it will be seen how the peakvibratory stress is concentrated more in the area of r₁ (FIG. 5 a) witha critical stress location shown as A, while the peak steady stress islocated closer to the r₂ zone with a critical stress location shown asB. Hence, the “D” shaped cutback of repair 38 helps in decoupling peaksteady stress and peak vibratory stress locations. With a “D” shapedcutback in place and appropriately located, two critical locations canbe well separated, thus making the blade repair scheme acceptable.

Referring to FIGS. 6 a and 6 b, other alternative shapes of the cutback38 are shown, in which the fillets r₁, r₂ are offset from the leadingedge 34 (although a similar configuration could be used on the trailingedge 36 as well). The fillets r₁, r₂ are offset from the edge bystraight portions as in FIG. 6 a, or by arcuate portions, as in FIG. 6b, or by a combination of both, etc. The straight portions of FIG. 6 amay be angled or perpendicular to the edge 34, 36, and may bequasi-straight, etc. In the instances of FIGS. 6 a and 6 b, the depth dincludes the offset (if any). The offset of FIGS. 6 a and 6 b may beused in larger blades, for instance.

The method to repair a damage blade in accordance with the presentdisclosure comprises identifying a damage on a leading and/or trailingedge of an airfoil of the blade. A cutback 38 is formed about the damagein the leading and/or trailing edge, the cutback shaped to comprise atleast a pair of fillets r₁, r₂ in the edge on opposite ends of thecutback, a depth d from the leading edge, and a length l in the leadingor trailing edge. As the skilled reader will appreciate, a d′ isselected to be suitable for the airfoil in question. For example, onlarger airfoils like turbofan fan blades, a d′=10d may be appropriate,while on smaller airfoils like high pressure compressor airfoils, it maynot be appropriate as d′ would be too large.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that changes may be made to the embodimentsdescribed without departing from the scope of the invention disclosed.For example, blades in any other suitable type of engines may berepaired with the cutback 38. Still other modifications which fallwithin the scope of the present invention will be apparent to thoseskilled in the art, in light of a review of this disclosure, and suchmodifications are intended to fall within the appended claims.

1. A method for repairing a blade in a gas turbine engine comprising:identifying a damage on an edge of an airfoil of the blade; forming acutback around the damage in the edge, the cutback shaped to comprise atleast a pair of fillets r₁, r₂ in the edge on opposite ends of thecutback, a depth d from the edge, and a length l along the edge.
 2. Themethod according to claim 1, wherein forming the cutback comprisesforming the fillets r₁, r₂ each with a different radius.
 3. The methodaccording to claim 1, wherein forming the cutback comprises spacing thefillets r₁, r₂. apart relative to one another in the cutback.
 4. Themethod according to claim 3, wherein spacing the fillets r₁, r₂. apartrelative to one another in the cutback comprises spacing the fillets r₁, r₂. apart with one of a generally straight edge and an edge having aradius of curvature substantially larger than r₁, r₂.
 5. The methodaccording to claim 1, wherein forming the cutback comprises forming thecutback with l/d=1 to 20, and r₁/d and r₂/d=0.2 to
 20. 6. The methodaccording to claim 1, wherein forming the cutback comprises forming thecutback with l being between 0.060″ and 3.00″; d being between 0.030″and 1.5″; and r₁, r₂ being between 0.030″ and 1.5″.
 7. The methodaccording to claim 1, wherein identifying a damage on an edge of anairfoil of the blade comprises identifying a damage in one of a leadingedge and a trailing edge, and wherein forming a cutback around thedamage in the edge comprises forming a cutback in the leading ortrailing edge.
 8. A blade in a gas turbine engine comprising: an airfoilhaving a leading edge and a trailing edge; and a cutback machined in atleast one edge among the leading and trailing edges at a location ofdamage, the cutback comprising a shape defined by at least a pair offillets r₁, r₂ on opposite ends of the cutback, a depth d from the edge,and a length l along the edge.
 9. The blade according to claim 8,wherein the fillets r₁, r². each have a different or same radius. 10.The blade according to claim 8, wherein the fillets r₁, r₂. are spacedapart by an edge in the cutback.
 11. The blade according to claim 10,wherein the edge spacing the fillets r₁, r₂. apart relative to oneanother in the cutback is one of a generally straight edge and an edgehaving a radius of curvature substantially larger than r₁, r₂.
 12. Theblade according to claim 8, wherein the cutback is defined by l/d=1 to20, and r₁/d and r₂/d=0.2 to
 20. 13. The blade according to claim 8,wherein the cutback is defined by l being between 0.060″ and 3.00″; dbeing between 0.030″ and 1.5″; and r₁, r₂ being between 0.030″ and 1.5″.14. A gas turbine engine comprising: at least one blade having a leadingedge and a trailing edge; and a cutback machined in at least one edgeamong the leading and trailing edges at a location of damage, thecutback comprising a shape defined by at least a pair of fillets r₁, r₂on opposite ends of the cutback, a depth d from the edge, and a length lalong the edge.
 15. The gas turbine engine according to claim 14,wherein the fillets r₁, r₂. each have a different or same radius. 16.The gas turbine engine according to claim 14, wherein the fillets r₁,r₂. are spaced apart by an edge in the cutback.
 17. The gas turbineengine according to claim 16, wherein the edge spacing the fillets r₁,r₂. apart relative to one another in the cutback is one of a generallystraight edge and an edge having a radius of curvature substantiallylarger than r₁, r₂.
 18. The gas turbine engine according to claim 14,wherein the cutback is defined by l/d=1 to 20, and r₁/d and r₂/d=0.2 to20.
 19. The gas turbine engine according to claim 14, wherein thecutback is defined by l being between 0.060″ and 3.00″; d being between0.030″ and 1.5″; and r₁, r₂ being between 0.030″ and 1.5″.